Editor template assumes all IHtmlContent implementations override ToString()

[dougbu]

The default HTML template for a POCO object examines the IHtmlHelper.Label() return value using !string.IsNullOrEmpty(label.ToString()). This condition assumes all IHtmlContent implementations override ToString(). Further the condition works only as long as ToString() returns string.Empty when the IHtmlContent implementation will no-op in its WriteTo() method.

This problem does not show symptoms when using the default Label() implementation. That method always returns either HtmlString.Empty or a TagBuilder. HtmlString.Empty.ToString() returns the empty string. TagBuilder.ToString() always returns the type's FullName and TagBuilder.WriteTo() always writes something - at least angle brackets and the tag name. So, things appear to work at the moment.

Example cases where the current assumptions would be invalid:

• We update TagBuilder to have a new TagRenderMode.Empty but still do not override ToString().
• In 2.0, we remove the HtmlString.ToString() override because it's redundant with HtmlString.Value and maintained only for back-compat.
• Users subclass HtmlHelper and override the GenerateLabel() method to return their own IHtmlContent implementation. That implementation does not satisfy our assumptions. (The assumptions aren't contractual or enforced of course.)

Side note: The default IHtmlHelper.Label() implementation rarely returns HtmlString.Empty but it definitely can do so. E.g. a property may have [Display(Name = "")].

[dougbu]

Taking because I've got the fix and previously discussed the issue w/ @Eilon

[dougbu]

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